High speed rotary lithographic printing plates require ink to be continuously applied to a rotating plate cylinder carrying a printing plate having an ink receptive image chemically formed thereon. The image is transferred from the plate cylinder onto a blanket cylinder which rolls against paper to be printed and impresses the image onto the paper. In order to maintain even pressure between the plate cylinder and the blanket cylinder and to provide a clear, crisp image without smearing, the areas of the printing plate which do not receive ink are appropriately treated to receive a thin coating of water or another dampening fluid. An appropriate thickness of dampening fluid, corresponding to the thickness of ink, is desired so that even hydraulic pressure results upon rolling contact between the plate cylinder and the blanket cylinder.
The plate cylinder is supplied with both ink and water during printing operations. The ink is supplied through a series of ink transfer rollers and the dampening fluid, which is usually water, with or without a small quantity of additives to adjust surface tension or other characteristics, is supplied through a series of dampening rollers.
In the past, the dampening roller systems have been two basic types. One type meters the amount of water using a ducting roller which periodically contacts a pan roller which rotates in a pan of dampening fluid to carry fluid to the ducting roller. The ducting roller then moves out of contact with the pan roller and contacts one of the transfer rollers for a period of time. The speed of the pan and ducting rollers and the dwell time determines the quantity of water transferred to the plate cylinder.
Another type of dampening system is a continuous supply dampening system in which the transfer rollers continuously and simultaneously communicate with both a pan roller and a transfer roller. The quantity of dampening fluid is metered by (1) setting the pressure at a contact line or "nip" point between the pan roller and a transfer roller, or (2) an adjustable speed control motorized pan roller, using a slip nip for metering. A continuous dampening system is normally desirable for efficient high speed rotary press operation.
In a dampening system using a ducting roller, a pan roller is partially immersed into a reservoir or pan of water or dampening fluid which is maintained at a substantially constant level. The pan roller rotates in the water or dampening fluid lifting a quantity of the fluid onto the roller. A ducting roller intermittently contacts the pan roller. It rolls along the pan roller surface for a desired period of time accepting a quantity of water on the ducting roller. The ducting roller then moves out of contact with the pan roller and into contact with an intermediate roller which is chrome plated or stainless steel, so that it is hydrophilic (i.e., water receptive). The intermediate roller accepts a quantity of water from the ducting roller onto its surface. The intermediate roller is maintained in constant rolling contact with a form roller which accepts a quantity of water from the intermediate roller and applies it to the printing plate which is wrapped around and rotates with the plate cylinder. In order to maintain an even thickness of water across the entire surface of the form roller, the intermediate roller may oscillate horizontally back and forth along its axis while it is in rolling contact with the form roller. The axial oscillating or vibrating action avoids "ghosting" which might otherwise occur when the lithographic plate depletes the form roller of water in certain areas on a repeated continuous basis. The oscillation of the water receptive intermediate roller acts to smear the water onto the form roller. This continuously replenishes an even layer of water across the entire surface so that the dry areas do not appear on the printed sheet.
It has previously been thought that dissimilarity between the soft fabric covered or rubber coated form roller and the hard chrome plated or stainless steel oscillating roller was desirable. Such an arrangement allowed the two rollers to slide relative to each other while they were simultaneously in rolling contact. Further, the water receptive and grease or ink repulsive characteristic of the chrome or metal surface of the oscillating roller was considered desirable to act as a barrier against ink transfer back up through the water dampening roller system. Ink contamination in the dampening fluid or the water source was reduced. For example, subsequent printing jobs with different colored inks could sometimes be printed without purging the entire water dampening system.
In a continuous dampening system, the metering of the amount of water is not accomplished through intermittent oscillation of the ducting roller, but rather is accomplished through appropriate adjustment of pressure at a nip point between the pan roller and the adjacent transfer roller. Typically, either the pan roller or the adjacent transfer roller had flexible rubber surfaces or other flexible polymeric surfaces which permitted adjustable pressure at the rolling contact line or the nip point. Adjustment of metering pressure was accomplished between the two flexible roller surfaces. Pressure between a transfer roller and an intermediate oscillating roller or between the oscillating roller and the form roller was generally fixed or set at a minimum contact level necessary for transfer of dampening fluid. It was generally thought to be undesirable to increase this pressure as it would increase friction and/or interfere with the relative sliding or oscillating motion of the rollers.
In another system as described in U.S. Pat. No. 3,902,417 issued Sep. 2, 1975 for a wetting system for rotary offset printing presses shows the use of an oscillating transfer roller which is positively driven in synchronism with a drive from the plate cylinder. The transfer roller, although ink receptive, as with copper or a hard nylon known as RILSAN, is provided with an unyielding surface for rolling engagement with a yielding form roller which slips slightly with respect to the plate cylinder. Although the pressure between the unyielding oscillating roller and the yielding surface of the form roller is adjustable to cause a depression into the form roller or a kneading effect, it has been found that the hydraulic forces between the oscillating roller and the form roller can increase significantly as the press speed increases and that without directly increasing the adjusted pressure between the rollers, the hydraulic pressure increases sufficiently to cause bowing at the centers of the rollers which results in excessive dampening fluid in the middle of the rollers and an absence of dampening fluid at the ends of the rollers which are supported by end bearings. Adjustable pressure between the rollers has not been found to be an adequate solution. The amount of pressure required at any given speed is not easily predictable and even if pressure adjustment alone would avoid the blanking at high speeds, press operation typically requires a significant amount of speed changes from set-up, initial runs, start-up production runs and shut-down phases of operation that a press operator might find himself continuously adjusting the pressure to avoid end blanking. Some complex systems of skewing or twisting the rollers with respect to each other in order to avoid the bowing and end blanking difficulty have been used in inking roller systems but have not been found to be sufficiently simple and operational to be truly effective. Moreover, skewing or roller twisting systems have not previously been successfully implemented in dampening roller systems.
One system attempted to use an oscillating roller which was interposed in the series of transfer rollers as an intermediate oscillating roller. It was driven in rotation only by frictional rolling contact and it was driven in oscillation only with a fragile internal groove and tab mechanism. This system was fraught with deficiencies and has apparently been abandoned altogether. It appears that balancing the need for pressure for rolling contact with the need for an absence of pressure to allow the fragile oscillating mechanism to function was one factor which doomed such a system to failure.
In a dampening system described in U.S. Pat. No. 4,949,637, issued to Keller on Aug. 21, 1990, the amount of water applied was metered through the use of multiple adjustable pressure nip points. The intermediate roller had a rubber surface to permit adjusting pressure, but it did not oscillate. A separate oscillating roller was held with a light spring tension either against the form roller, against the transfer roller, or two oscillating rollers were held, one against the form roller and one against the transfer roller. The oscillating roller had a rubber surface, 95 to 100 Shore A Durometer, and was rotated through rolling contact friction, not gear driven. The oscillating mechanism was a fragile mechanism small enough to fit inside the roller itself, interposed between the roller shaft and the roller itself. This system worked and continues to work well for small size presses, less than about 22 inches wide, but adequate pressure and oscillation for larger size presses is increasingly difficult to maintain, especially with very high speed presses.